docs(material/theming): add tokens section to our docs

### Documentation Feedback

Add documentation to material.angular.io to list the tokens that our components expose.

#### Design

There are still some open questions that the team should discuss.

- Should we write a description for each token?
- Critiques:
- A unique description for every token may be too much data thrown at the user which the user might mentally filter out when looking through our docs.
- Many tokens are self-explanatory. It seems silly to write `"selected-checkmark-color": The color of the checkmark when the checkbox is selected`.
- A more useful approach may be to provide a subsection that explains terminology that is obscure to most users. For example, the section could describe the following:
- What `state-layer` commonly refers to.
- Listing the different states - `active`, `pressed`, etc. Material has [a page dedicated to this](https://m2.material.io/design/interaction/states.html#usage) we could also link to.
- Where should this token list live?
- IMO under the "API" tab for each component makes the most sense.

This design satisfies the following pain point for our users:

- CSS variables are difficult to identify / discover.

### Affected documentation page

https://material.angular.io/components/autocomplete/api

影響を受ける Autocomplete API ドキュメントページ（material.angular.io/components/autocomplete/api）から始め、コンポーネント API ドキュメントがどのように構成されているかを確認します。トークンの説明、用語、配置に関する未解決の疑問をチームと解決します。公開されている CSS トークンが適切なコンポーネント API ドキュメントから見つけられ、合意された用語ガイダンスがあることを完了条件とします。

索引モデルが issue の本文から書いたものです。